He was a giant of 20th-century art, revered for his sensitivity to line and colour. Now an extensive interview in which Henri Matisse freely shares his thoughts on everything from drawing to depression is to be published for the first time since he angrily blocked its publication more than 70 years ago.

In 1941, in an interview that spanned several days in Lyon and Nice, Matisse spoke with remarkable candour to an art critic, who planned to publish the transcript in a book. But the artist changed his mind and refused to allow the project to go to press.

Matisse was outraged partly by the editing, which reduced the text to 260 pages, a decision influenced by wartime paper shortages and cost-cutting. "You want to mutilate my work," he fumed. "I do a thing well or not at all. There are around 310 pages … do you wish to accept them in their entirety and publish them?"

Disagreements with the publisher, Skira, extended to the choice of paper. But he also felt that some things should not be made public. Matisse pored over the text, rewriting passages, and completely rejecting some 25 pages, including disparaging comments about collectors, whom he "always finished up detesting". He spoke of dismay in finding that one collector had put a painting in his bathroom, saying: "Americans are like children who do the first thing that comes into their head." He was also dismissive of dealers, observing: "To be a good art dealer … you just have to know how to sell."

Elsewhere, the artist confided: "I'm rather inclined to depression and sometimes see everything in black … The biggest worry was losing my love of work. I can put up with the blackest despair by whistling or singing."

It has taken until now to persuade his family that the interviews should finally be released. Chatting with Henri Matisse: The Lost 1941 Interview will be published in September.

The interview languished among the papers of Pierre Courthion, a Swiss art critic, whose archives were acquired in the 1980s by the Getty Research Institute. Biographers have dipped into the material, but this marks its first publication – a collaboration between the Tate and Getty.

Its editor, Professor Serge Guilbaut, said: "To publish a book rejected by its author is not an easy task. It took not only many years but also many conversations …

"Claude Duthuit, grandson of Matisse, was not keen to have all this published because this would contradict Matisse's wishes. But, when he realised that the Getty had the definitive text ready to be published, he thought that it was an interesting historical document. He wrote an introduction."

Guilbaut added: "This interview was known by some specialists of Matisse but never really quoted at length. Many passages, stories, criticisms and even analysis by Matisse of his work, like colour, Haiti, Morocco, have not been really discussed. [There are] lots of original discussions … and many details about his life, his problems and hopes, directly from the mouth of the master."

Matisse, who died in 1954, explored the expressiveness of colour. Guilbaut's preface notes that the interviews shed new light on his technique, with Matisse describing colours as singing together "like a musical chord" and likening a painting to a symphony.

The artist spoke of colour's expressive qualities: "For me, a colour is a force. My pictures are made up of four or five colours that collide with one another, and the collision gives a sense of energy. When I put green, it doesn't mean grass. When I put blue, it doesn't mean sky."

He also recalled being given a paintbox while convalescing from appendicitis aged 21 and buying a book called How to Paint: "That was the seed. It was bound to grow – the bud had to blossom. Since then, I've hardly given a thought to anything else, just painting. It sprang up like a weed; who knows where it came from."